1. Field of the Invention
The present invention relates to a high strength steel sheet having 340 MPa or higher strength and giving excellent stretch flanging performance, ductility, shock resistance, surface properties, and other characteristics, and relates to a method for manufacturing the same.
2. Description of Related Art
Steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets are press-worked in various shape members for use in the fields of automobiles, household electric appliances, industrial machines, and the like. In recent years, manufacturers of automobiles and other products have increased their use rate of high strength steel sheets responding to the need of weight reduction.
The high strength steel sheets have, however, problems such as the stretch flanging cracks occurred when the high strength steel sheets having 340 MPa or higher strength are treated by burring, the workability issue such as insufficient ductility of high strength hot dip zinc-coated steel sheets having 440 MPa or higher strength, and the issue of insufficient shock resistance which is important to secure safety on collision. Those types of high strength steel sheets having 340 MPa or higher strength are manufactured using the base carbon steel being adjusted in carbon equivalent to 0.05 to 0.2 wt. % C, adding precipitation-strengthening elements such as Ti, Nb, and V responding to the strength thereof. When, however, the steels of these compositions are hot-rolled, cracks likely occur, which degrades the surface properties to significantly reduce the production yield.
As the technologies for improving the workability of high strength steel sheets, JP-B-61-15929 and JP-B-63-67524, (the term xe2x80x9cJP-Bxe2x80x9d referred herein signifies xe2x80x9cExamined Japanese Patent Publicationxe2x80x9d), for example, disclose the method to improve the balance of strength and ductility, the breaking elongation (ductility), and the toughness by controlling the cooling speed after hot-rolling and the coiling temperature. As the technology to improve the stretch flanging performance, Japanese Patent No. 2555436 discloses the method for manufacturing steel sheet having strengths of from 500 to 600 MPa and having excellent stretch flanging performance, which steel sheet is prepared by hot-rolling a Ti-added steel, by cooling the steel sheet at cooling speeds of from 30 to 150xc2x0 C./sec, and by coiling the steel sheet at temperatures of from 250 to 540xc2x0 C. to establish a (ferrite +bainite) structure. JP-B-7-56053 discloses the method for manufacturing hot dip zinc-coated steel sheet having strengths of from 450 to 500 MPa and having excellent stretch flanging performance, which steel sheet is prepared by cooling a hot-rolled steel sheet at cooling speeds of 10xc2x0 C./sec or more to establish a (ferrite+pearlite) structure. JP-A-4-88125, (the term xe2x80x9cJP-Axe2x80x9d referred herein signifies xe2x80x9cUnexamined Japanese Patent Publicationxe2x80x9d), discloses the method for manufacturing steel sheet having strengths of from 500 to 700 MPa and having excellent stretch flanging performance, which steel sheet is prepared by hot-rolling a Ca-added steel at temperatures of from (Ar3 transformation point+60xc2x0 C.) to 950xc2x0 C., by cooling the steel sheet within 3 seconds after completed the hot-rolling down to the temperature range of from 410 to 620xc2x0 C. at cooling speeds of 50xc2x0 C./sec or more, by cooling the steel sheet in air, and by coiling the steel sheet at temperatures of from 350 to 500xc2x0 C. to establish a (ferrite+pearlite) structure. JP-A-7-54051 discloses the method for manufacturing high strength hot dip zinc-coated steel sheet having excellent stretch flanging performance and ductility, which steel sheet is prepared by hot-rolling a Nbxe2x80x94Ti added steel at temperatures ranging from 850 to 1,000xc2x0 C., by cooling the hot-rolled steel sheet down to 600xc2x0 C. at average cooling speeds of 40xc2x0 C./sec or more, by further cooling the steel sheet at average cooling speeds of 30xc2x0 C./sec or less, by coiling the steel sheet at temperatures of from 400 to 550xc2x0 C., and by applying hot dip zinc-coating.
The methods described in these prior arts, however, have problems of unable to completely prevent the stretch flanging cracks occurred during burring treatment, of not necessarily unable to assure excellent shock resistance, and of giving insufficient coil shape when the coiling temperature becomes to below 400xc2x0 C. caused from low ductility. For the case of hot dip zinc-coated steel sheet, there are several problems on attaining satisfactory ductility, including the problems of limitation on added amount of Si which is effective to improve ductility, and of unable to apply (ferrite+martensite) structure which is effective in ductility improvement for the use requiring high yield ratio.
The present invention was completed to solve the above-described problems, and an object of the present invention is to provide a high strength steel sheet having 340 MPa or higher strength and providing excellent stretch flanging performance, ductility, and shock resistance, and giving a sufficient coil shape and favorable surface properties even under hot dip zinc-coating treatment.
The object of the present invention is attained by a high strength steel sheet consisting essentially of 0.04 to 0.1% C, 0.5% or less Si, 0.5 to 2% Mn, 0.05% or less P, 0.005% or less 0, 0.005% or less S, by weight, having 10 xcexcm or less of average ferritic grain size, and 20 mm/mm2 or less of generation frequency A, which generation frequency A is defined as the total length of a banded secondary phase structure observed per 1 mm2 of steel sheet cross section along the rolling direction thereof.
The high strength steel sheet is prepared by a manufacturing method comprising the steps of: hot-rolling a continuously cast slab having the composition described above at temperatures of Ar3 transformation point or above directly or after reheating thereof; and cooling the hot-rolled steel sheet within 2 seconds down to the temperatures of from 600 to 750xc2x0 C. at cooling speeds of from 100 to 2,000xc2x0 C./sec, followed by coiling the cooled steel sheet at temperatures of from 450 to 650xc2x0 C.
In particular, to further improve the ductility of high strength hot dip zinc-coated steel sheet having strengths of 440 MPa or more, it is preferred to apply a manufacturing method comprising the steps of: hot-rolling a steel slab consisting essentially of 0.01 to 0.3% C, 0.7% or less Si, 1 to 3% Mn, 0.08% or less P, 0.01% or less S, 0.08% or less sol.Al, and 0.007% or less N, by weight, at temperatures of Ar3 transformation point or above; cooling the hot-rolled steel sheet within 2.5 seconds down to the temperatures ranging from above 500xc2x0 C. to 700xc2x0 C. at average cooling speeds of 100xc2x0 C./sec or more, followed by coiling the cooled steel sheet; and pickling or cold-rolling after pickling the coiled steel sheet, then annealing thereto in a continuous hot dip zinc-coating line at temperatures of 720xc2x0 C. or above to perform the zinc coating.
For completely preventing the degradation of surface properties caused from the cracks generated during hot-rolling, it is preferred to apply a manufacturing method comprising the steps of: hot-rolling a continuously cast slab consisting essentially of 0.05 to 0.2% C, 0.15% or less Si, 0.4 to 2.0% Mn, 0.025% or less P, 0.005% or less 0, 0.01% or less S, 0.006% or less N, and 0.004% or less Sn, by weight, and having Mn/Sxe2x89xa750 at temperatures of Ar3 transformation point or above directly or after reheating the continuously cast slab; and cooling the hot-rolled steel sheet down to the temperatures of from 400xc2x0 C. to 700xc2x0 C. at cooling speeds of from 20 to 2,000xc2x0 C./sec, followed by coiling the cooled steel sheet.